1. Field of the Invention
The present invention relates to a power semiconductor device, and particularly to a power semiconductor device provided with a vertical power device.
2. Description of the Background Art
As power devices, there have been power diodes, power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors) and the like, and a structure that has only one or a plurality of such device(s) (chip(s)) as well as one package accommodating the device(s) together with desired wiring has been generally referred to as a power semiconductor device.
A “power cycle life” is one of indexes that are used when reliability of the power semiconductor devices are considered. This power cycle life represents a life determined in connection with a wire that is joined as a part of interconnections to an electrode of a chip, and particularly is a life that expires when peeling or breakage of the wire occurs on or near a joining interface of the wire due to frequent changes in temperature according to operations. For example, in the IGBT, the power cycle life depends on a temperature (Tj) of a region where an emitter electrode is joined to an emitter wire, and increases with decrease in ΔTj(=Tjmax.−Tjmin.). It has also been known that the power cycle life increases with decrease in Tjmax. even when ΔTj is constant. For example, Japanese Patent Laying-Open No. 2004-363327 has disclosed a power device developed in view of improvement of the above power cycle life.
For example, in a power semiconductor device having a plurality of wires (emitter wires) connected to emitter electrodes of an IGBT chip, a current gathers at connection portions of the emitter wires so that a temperature Tja of the emitter wire connection portions is higher than a temperature Tjb of their surrounding region. Consequently, values of Tjmax. and ΔTj increase, resulting in a problem that the power cycle life decreases.